Implantable devices that provide electrical stimulation to selected chambers of the heart have been developed in order to treat a number of cardiac disorders. A pacemaker, for example, is a device which paces the heart with timed pacing pulses, most commonly for the treatment of bradycardia where the ventricular rate is too slow. Implantable devices may also be used to treat cardiac rhythms that are too fast, with either anti-tachycardia pacing or the delivery of electrical shocks to terminate atrial or ventricular fibrillation.
Implantable devices have also been developed that affect the manner and degree to which the heart chambers contract during a cardiac cycle in order to promote the efficient pumping of blood. In a properly functioning heart, contraction of each atrium or ventricle is synchronized with the contralateral atrium or ventricle. Without such synchronization, the heart's pumping efficiency is greatly diminished. To treat patients suffering from inefficient or unsynchronized pumping of the heart, CRT devices, which provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of atrial or ventricular contractions, have been developed.
A common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a programmed atrioventricular (AV) delay interval with respect to the detection an intrinsic atrial contraction or delivery of an atrial pace. With such CRT, an electrode of a right ventricular lead is placed in contact with the right ventricle, typically at the apical wall, and a left ventricular lead containing an electrode for pacing the left ventricle is typically placed in a vein of the coronary sinus overlying the left ventricle.
Site selection for coronary sinus (CS) left ventricular (LV) lead placement, whether for biventricular pacing or for left ventricular pacing, may be important for effecting patient response to CRT. One criteria that has been proposed for CS LV lead placement is electrical timing of the CS sites, where the latest site of electrical activation has been proposed as being the best site for placement of the LV lead. However, there appears to be little or no data supporting the idea that lateness of electrical activation during intrinsic or paced rhythms will predict a site for LV lead placement that will result in a favorable response to CRT.